LazyImage.m

+ (UIImage *)imageWithBlock:(UIImage *(^)(void))block {
    MRLazyImage *lazyImage = [(MRLazyImage *)[self alloc] initWithBlock:block];
    return (UIImage *)lazyImage;
}

- (UIImage *)image {
    if (!_image && self.block) {
        _image = self.block();
        self.block = nil;
    }
    return _image;
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    invocation.target = self.image;
    [invocation invoke];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.image methodSignatureForSelector:sel];
}